Gaseous waste products from the combustion of fuels pose serious health and environmental problems. Exhaust gases from hydrocarbonaceous fuel burning sources such as stationary engines, industrial furnaces, industrial processes, etc., contribute significantly to air pollution, and the exhaust gases of automobile internal combustion engines have been recognized as a principal source of air pollution. In recent years there has been increasing interest, in view of the large number of automobiles traveling our roads, particularly in urban areas, in controlling the amount of gaseous waste products from automobile exhausts.
Automotive catalytic converters containing exhaust gas catalysts have more or less enabled automobiles to meet current standards established by government agencies to convert a substantial portion of hydrocarbons and carbon monoxide to water and carbon dioxide and the NO.sub.x gases to nitrogen and oxygen and/or water. A wide variety of metals and metal oxides, either alone or in combination, supported on various substrates have been utilized. In recent years, most exhaust gas catalysts have employed a combination of noble metals, particularly platinum, rhodium and/or palladium, as the active materials of the catalyst.
Typically, exhaust gas catalysts comprise a relatively low porosity ceramic support with a transition alumina coating having a high surface area. The underlying ceramic support is generally prepared by sintering a mold of clay or other ceramic material at a high temperature to impart density and strength. This, however, generally results in a support having a very low surface area. Consequently, the ceramic support must be coated with another material having a much higher surface area to contain the catalytic components. The procedure of depositing a high surface area "washcoat", as such coating is generally known, onto a low surface area ceramic support is disclosed in, for example, U.S. Pat. Nos. 2,742,437 and 3,824,196. The ceramic supports may be provided in any shape, but typically they are in the form of pellets or a honeycomb-type shape commonly known as a monolith.
Gamma-alumina is often used as the washcoat in such exhaust gas catalysts. Although a gamma-alumina washcoat imparts a relatively high surface area to an exhaust gas catalyst, it results in number of undesirable effects. Often the washcoat does not adhere well to the underlying ceramic support under severe thermal stress, or has a level of thermal expansion incompatible with the ceramic support. In addition, gamma-alumina or transition-alumina washcoats are thermodynamically unstable alumina phases. Eventually this unstable gamma-alumina phase transforms to a thermodynamically stable alpha-alumina phase; however, in the process of transforming, the alumina loses surface area and traps the catalytic metals and may even change their oxidation state, rendering the less effective or ineffective.
Conventional washcoated exhaust gas catalysts also require a time-consuming, tedious, cost-ineffective, multi-step preparation procedure. This procedure includes preparation of the support, preparation of the washcoat itself, application of the washcoat onto the support and impregnation of all the catalytic and promoter components individually or collectively on to the supported washcoat.
Although wash coated exhaust gas catalysts have acceptable initial light-off temperatures, with age their light-off temperatures often increase, sometimes rapidly. Light-off temperature ("T.sub.50 ") is the temperature at which an exhaust gas catalyst begins to convert 50 percent of the waste products of the exhaust gas into carbon dioxide, water, nitrogen and oxygen. Thus, when an automobile is initially started and for the time until the catalyst reaches its light-off temperature, most of the exhaust gases are not catalytically treated but are simply emitted into the atmosphere.
Stable catalytic activity is becoming a critical requirement with automotive exhaust gas catalysts. Conventional exhaust gas catalysts lose approximately half of their activity relatively rapidly, i.e., during the first 12,000 miles of use. Often washcoated exhaust gas catalysts actually physically deteriorate. New government standards for catalytic converters containing exhaust gas catalysts have much stricter longevity requirements, in that such catalytic converters must perform efficiently for much longer periods of time, i.e., 50,000-100,000 miles of use.